I am making ricotta and it is not forming curds. I used a gallon of whole milk with salt then took it off heat when temp. Reached 180, added a bot more than 1/3 cup white vinegar and let it sit. How can I fix this?

thanks - I did put it all back in the pot, went a bit over 180 this time and added another 1/3 c of vinegar. Kept it on the heat for a bit until I saw curds - It produced about 4 cups and it tastes fine. Yay!